Aviation regulators worldwide laid down a stark challenge for Boeing to prove that its grounded 737 Max jets are safe to fly amid suspicions that faulty software might have contributed to two crashes that killed 346 people in less than six months.

In a key step toward unearthing the cause of the Ethiopian Airlines crash, flight recorders from the shattered plane arrived Thursday in France for analysis, although the agency in charge of the review said it was unclear whether the data could be retrieved. The decision to send the recorders to France was seen as a rebuke to the United States, which held out longer than most other countries in grounding the jets.

Boeing executives announced that they had paused delivery of the Max, although the company planned to continue building the jets while it weighs the effect of the grounding on production.

In Addis Ababa, angry relatives of the 157 people who were killed Sunday stormed out of a meeting with airline officials, complaining that they were not getting enough information.

The U.S. Federal Aviation Administration grounded the planes Wednesday, saying regulators had new satellite evidence that showed the movements of Ethiopian Airlines Flight 302 were similar to those of Lion Air Flight 610. That flight crashed into the Java Sea off Indonesia in October, killing 189 people.

The Max jets are likely to be idle for weeks while Boeing tries to assure regulators around the world that the planes are safe.

At a minimum, aviation experts say, the plane maker will need to finish updating software that might have played a role in the Lion Air crash. Regulators will wait for more definitive evidence of what caused both crashes. Some industry officials think the plane maker and U.S. regulators may be forced to answer questions about the plane’s design.

Boeing said it supports the grounding of its planes as a precautionary step, while reiterating its “full confidence” in the safety of the 737 Max. The company has previously characterized the software upgrades as an effort to make a safe plane even safer. Engineers are making changes to the system designed to prevent an aerodynamic stall if sensors detect that the jet’s nose is pointed too high and its speed is too slow.